Compactible fused and atomized metal powder



United States Patent 3,498,782 COMPACTIBLE FUSED AND ATOMIZED METALPOWDER Donald R. Spink, East Amherst, and Allen C. Goodrich, EastAurora, N.Y., assignors to Amax Specialty Metals, Inc., a corporation ofDelaware No Drawing. Filed Feb. 18, 1966, Ser. No. 528,390 Int. Cl. B22f1/00 U.S. Cl. 75-.5 4 Claims ABSTRACT OF THE DISCLOSURE Metal powdersproduced by fusing and a-tomizing metals selected from Groups 4, 5 and6, the rare earths, and the actinium series of the periodic table andtheir alloys are made highly compactible by reducing their relativelyhigh energy state in the as-produced condition to a lower energy stateaccompanied by the discharge of energy in the form of heat. Thisreduction in energy state is accomplished by controlled annealing or bya plurality of compacting and recrushing steps, or a combinationthereof.

This invention relates to compactible metal powders and a process formaking powders compactible. More particularly this invention relates'xto plasma-arc produced powders of high melting metals such asthose selected from Groups 4, 5, and 6, the rare earths and the actiniumseries of the periodic table and a process for making these metalpowders more compactible.

The necessity for metal parts capable of operating under severeconditions, such as in nuclear reactors where such parts must be highlycorrosion resistant and must possess high temperature resistance andmechanical strength, has increased the demand for parts formed fromalloys of such high melting metals as zirconium, titanium, tungsten,tantalum, uranium, and niobium. Fabrication of these metals and theiralloys has been restricted due to ease of contamination of these metalsby elements such as oxygen, nitrogen, and iron, and the difiiculty inworking the metals by conventional metal working techniques. Forexample, parts formed from zirconium based alloys frequently exhibitundesirable anisotropic properties as well as low corrosion resistancewhen formed by conventional metal working techniques. The economicaloptimization of corrosion properties, physical properties andfabrication of the aforementioned metals and their alloys have beenshown to lie in the use of the powder metallurgical techniques, andthese are an object of this invention.

Alloys of the aforementioned group of metals, pan ticularly zirconiumalloys, are frequently used in the alpha stabilized form. Alphastabilized alloys are those in which the alpha phase is stabilized, byalloy additions, to temperatures above that which it normally survives.-In the alpha stabilized form the effect of conventional metal workingand heat treating operations on the mechanical and chemical propertiesof the alloy is readily apparent. An alpha stabilized alloy comprising1.2 to 1.7 weight percent tin, 0.07 to .20 weight percent iron, 0.05 to0.15 weight percent chromium, and 0.03 to 0.08 weight percent nickel,and referred to in the industry as zircaloy 2, is particularly suitablein regard to neutron absorption, corrosion resistance and mechanicalstrength. However, when prepared by conventional metal workingtechniques this alloy may exhibit mechanical weakness and poor corrosionresistance which could cause premature failure in stressed applications.Using conventional techniques zircaloy is frequently forged starting at1700 to 1800 F. for 12 to 16 dia. ingots. Working is completed wellabove the 1450 F. so that a condition of slow cooling from 3,498,782Patented Mar. 3, 1970 the apha-beta phase is present during theproduction of forged bars.

The mechanical weakness may be associated with a relatively high degreeof anisotropy of properties (i.e. the finished articles have differentchemical and physical properties in one direction than in otherdirection). This slow cooling tends to form a microstructu-re of largealpha zirconium grains of relatively low alloy content surrounded by anetwork of coarse grains of intermetallic compounds and thismicrostruct-ure can exhibit anisotropic properties, especially inobjects having thin cross sections.

Moreover such a microstructure has been observed to yield poorresistance to corrosion apparently due to the intermetallic compounds.For example, the precipitation of iron and chromium intermetalliccompounds from slowly cooled alloys was found to be the cause ofintergranular corrosion in the heat affected zone of welds made incommercial zirconium of high impurity content.

Heat treatment is required to affect a resolution of the offendingintermetallic compounds in order to avoid the undesirable side eifectswhich may occur when such metals as zirconium, hafnium, titanium and thelike are worked by conventional techniques. It appears to be necessaryto change at least a major portion, if not all of the structure, to thebeta phase since the beta phase has a greater solubility for thealloying elements. Rapid quenching from the beta phase temperatureregion is performed to prevent the segregation of intermetallics whichmay be subsequently worked into stringers which extend through thegrains for considerable distances (as much as inch or more) causinganisotropic properties and areas of reduced corrosion resistance.

An example of such heat treatment is disclosed in U.S. Patent 2,894,866to Picklesimer wherein a procedure is set forth for preventing theformation of intermetallic stringers and anisotropic properties duringthe fabrication of alpha stabilized zirconia based alloys. This processincludes performing major size reduction at :a malleabilizingtemperature excluding the alpha plus beta range, heating the workpieceto a temperature above 970 C. {beta range) for at least approximately 30minutes, cooling the workpiece rapidly, working at a temperature belowapproximately 500 C. to reduce the cross sectional area of the workpieceby at least 20 percent, annealing at a temperature from approximately700 C. to 810 C. for at least approximately 15 minutes and finallycooling the article thus fabricated. This involved procedure involvesmany possibilities for the introduction of undesirable interstitialelements and other contamination. Moreover this procedure does notreadily lend itself to the production of intricate final shapes normallyrequired for nuclear reactor or other applications.

Powder metallurgy offers an attractive alternate approach to theintricate shaped hardware of many industries. In the fabrication ofzircaloy 2 and other metals, by powder metallurgy techniques, theconstituents may be blended to the desired homogeneity, pressed into anintermediate high density green compact which requires a minimum of hotor cold working and annealing to provide the desired finished shape. Thepowder metallurgical technique provides a simpler method which mayrequire less costly fabricating equipment. The application of thispowder metallurgical technique and using fused and atomized particleswhich have been rapidly quenched from the beta phase excludes theformation of intermetallic stringers and coarse preferentially orientedcrystals by introducing only fine crystals which are randomly oriented.A final shape more resistant to corrosion, of high purity, havingimproved mechanical properties and a simplified fabricating techniqueare advantages which result from the availability of the compactiblemicrospherical powder which is a subject of this invention. Theapplication of plasma produced microspherical powders which had adesirable shape and chemical purity was, prior to this invention,impractical because of the poor compactibility (low compact greenstrength and low compact density) of this material.

Accordingly, it is an object of this invention to provide metal powderproduced from high melting metals such as those of Groups 4, 5 and 6,the rare earths, and actinium series of the Periodic Table and theiralloys, which is readily and easily compacted into a dense, homogeneous,isotropic shape.

Another object is to provide a fused and atomized metal powder which canbe formed into a compact which has high green strength.

A further object is to provide a process for improving compaction ofmetal powders at moderate pressures.

A still further object of this invention is to provide a process formaking compactible metal powder from fused and atomized metal powders.

Various other objects and advantages will appear from the followingdescription of the embodiments of this invention, and the novel featureswill be particularly pointed out in connection with the appended claims.

We have found that metal powders formed by fusing and atomizin-g metalsselected from Groups 4, 5, and 6, the rare earths, and the actiniumseries of the Periodic Table can be treated in accordance with thisinvention to form highly compactible metal powders which are capable ofbeing formed by powder metallurgical techniques into articles possessingexcellent chemical and mechanical properties.

The metal powders treated in accordance with this invention are producedfrom Groups 4, 5, and 6, the rare earths, and the actinium series of thePeriodic Table and include such metals as zirconium, hafnium, titanium,tungsten, tantalum, niobium, lanthanum, thorium, uranium, and cerium.The metal powders are produced by fusion and atomization, preferably byplasma jet techniques. In producing metal powder by plasma jettechniques, the metal or alloy to be treated may be introduced as ashape, such as a rod or bar, or as a powder, into a stream of inert gasthat has been heated to such a temperature that it is dissociated orionized. The ionized gas sweeps over the metal shape melting the surfacemetal and carrying off the molten metal asdroplets in the gas stream.The metal droplets are subsequently atomized and extremely rapidlyquenched and the resultant product is in the form of uniformly shapedmicrospheres which may range in size from about +60 mesh to about 325mesh.

There are several advantages to using metal powder produced by plasmatechniques. First, metal alloys, as well as pure metals, may beconverted into powder efiiciently and economically. Other methods ofproducing alloy powders are rather cumbersome, and usually require longdiffusion treatments to insure complete alloying. Secondly, the metalpowder is of high purity and uniform shape and structure due to themethod of forming the metal droplets and their rapid quenching. Theresulting particles, known as microspheres, are less susceptible tosurface take-up of impurities because the uniform spherical shape of themicrospheres provides low surface to mass ratio. The spherical shape ofthe particles and the method of forming the microspheres avoids crystalorientation that could contribute to anisotropy. Prior to this inventionthe use of fused and atomized metal powders of metals from Groups 4, 5,and 6, the rare earths, and the actinium series of the Periodic Tablehas been restricted since such powders are only moderately compactibleand compacts made from these metal powders are extremely weak and mayrequire pre-sintering before they are capable of being handled. On theother hand compacts made from fused and atomized metal powders whichhave been processed according to this invention are highly compactibleand the compacts, before sintering, may achieve more than 97 percent ofthe theoretical density of the component metal.

The relatively uncompactible nature of the fused and atomized powders ofthe metals of Groups 4, 5, and 6, the rare earths, and the actiniumseries of the Periodic Table is attributed to the high energy state inwhich the powder particles exist in the as-produced condition. This highenergy state is presumably due to the extremely rapid quenching of themolten droplets of metal that occurs after fusing and atomizing themetal during the process for producing fused and atomized metal powder.We have found that lowering the energy state of the particles byannealing or cold working the powder in accordance with this inventionthe particles become highly compactible. This changing of the particlesfrom the high energy state to a low energy state is marked by a releaseof the energy in the form of heat and is accomplished withoutdeleteriously affecting the chemical and physical properties of themetal or alloy powder so treated.

In the practice of one embodiment of this invention, fused and atomizedmetal powders are annealed under an inert condition at a temperatureabove 300 C. but below the temperature at which the crystals change to aless corrosion resistant state. Inert conditions, as used throughoutthis specification, mean conditions wherein the environment surroundingthe particular metal or alloy being annealed is substantially unreactivewith that metal or alloy. Such an inert condition exists for examplewhen annealing in a vacuum or in an atmosphere of argon. However thisterm includes annealing a metal or alloy in any atmosphere which isunreactive with the material being treated and may include annealing inair in some cases. The temperature varies with the metal or alloy beingannealed. For example when using zirconium and zirconium alloys thetemperature at which the crystals change to a state of poor corrosionresistance is approximately 870 C. Therefore when annealing zirconium orits alloys, such as zircaloy 2, according to this invention, theannealing temperature is maintained between 300 and 870 C. The upperannealing temperatures are determined from experience and by a study ofthe phase diagrams of the particular metal or alloy being treated.

The exothermic nature of the change that is coincident with the changeto greater compactibility (higher green strength and density ofcompacts) has led to the following theory. The rapidly quenched, fusedand atomized particles are in a high energy state. On heating andholding at temperatures above 300 C. (more particularly at a temperatureof approximately 780 C. for zirconium alloys such as zircaloy 2) anexothermic reaction accompanies the change to a lower energy state whichis more amenable to compacting. This lower energy state can also beachieved by cold working.

Thus, although it is the preferred embodiment of this invention toanneal the microspheres in order to aifect the change of crystallineenergy state from the uncompactible high energy state to the compactiblelow energy state, it is within the scope of this invention to affect thechange by a multiplicity of compacting and crushing steps. I. is alsowithin the scope of this invention to lower the particle energy level bya combination of annealing and compacting and crushing steps.

The compacts are usually formed at a pressure of between 50,000 p.s.i.and 200,000 p.s.i. using conventional powder metallurgical techniquesand equipment.

Compactibility was determined by measuring the volume and weight of agreen compact using ASTM procedure (D70-52). Compact green strength wasdetermined by the following procedure. Five gram samples of the metalpowder being tested were pressed in a cylindrical mold having a diameterof /2 inch. These cylinders (not less than 5 per test) were dropped froma height of 36 inches into a 2000 ml. stainless steel beaker having afirmly supported flat bottom. The contents of the beaker were carefullypoured onto an 8 mesh screen. The pieces retained on the 8 mesh screenwere weighed. This weight when divided by the total sample weight andmultiplied by 100, resulted in the compact green strength for thearticle being tested.

The objects and advantages of this invention are shown in greater detailin the following examples and by reference to the appended claims.

The following example illustrates the increase in powder compactibilityobtained when fused and atomized metal powders are treated according tothis invention.

EXAMPLE 1 A fused and atomized metal powder, formed by conventionalplasma jet technique, such as described above, was prepared from azirconium alloy comprising, in addition to zirconium, 1.21.7 weightpercent tin, .05-.l weight percent chromium, .07.20 weight percent iron,and .03-.08 weight percent nickel.

The zirconium alloy metal powder had a mesh size ranging from +60 to--325 US. Standard Mesh.

Five gram samples of the fused and atomized zirconium alloy powder werepressed at various pressures to test the compactibility of the powder inthe as-produced condition. Tests were conducted on the as-producedpowder both with and without a die lubricant The die lubricant was apowdered fat manufactured by the Capital City Products Company,Columbus, Ohio and sold under the mark Sterotex. The results are shownin Table I.

TABLE I.GREEN" STRENGTH OF COMPACIS FORMED FROM AS-PRODUCED ZIRCONIUMALLOY FUSED AND ATOMIZED POWDER Percent compacted The results of Table Ishow that even at pressures of 200,000 p.s.i.g., using die lubricant,the compactibility of the as-produced powder is relatively poor.

To improve compactibility, samples which had been pressed at 150,000p.s.i.g. without die lubricant were recrushed and recompacted anadditional four times. The compacts were tested for green strength aftereach recompaction. Recrushing may be carried out by any conventionalmeans and although the compact may be reduced to a coarser powder thanthe original metal powder, it is preferred that crushing be continueduntil the compact is reduced to powder of substantially the same meshsize as the original powder. Recompacting was carried out at 150,000p.s.i.g. Table II shows the improvement in powder compactibility thatwas obtained by mechanically working the fused and atomized metal powderof Example 1.

TABLE II.EFFECT OF CRUST-TING AND RE COMPACT- ING 0N FUSED AND ATOMIZEDMETAL POWDER Number of recrushing and recompacting cycles:

Green strength (percent compacted) As can be seen from the results ofTable II, fused and atomized zirconium alloy powder, which is onlymoderately compactible in the as-produced condition, forms compacts ofhigh green strength which are at least 99 percent compacted after fourrecrushing and recompacting cycles.

The following example illustrates the increase in powder compactibilitywhen fused and atomized metal powder is annealed in accordance with thisinvetion.

EXAMPLE 2 To illustrate the effect of annealing on the compactibility ofthe microspherical powder, samples of zirconium microspherical powder,as produced in Example 1, were annealed in the following manner. Themicrospherical powder was placed in a cool annealing furnace C.) and thefurnace was purged with argon to exclude oxygen. The temperature of thefurnace was gradually increased to 775 C. :25" C. The temperature wasincreased gradually because at approximately 600 C. the power in thefurnace began to give off energy in the form of heat and consequentlyless heat input was needed in order to raise the temperature of thepowder to the desired annealing temperature. It was observed that if thefurnace temperature increase was too rapid the exothermic nature of thefused and atomized powder caused the furnace temperature to rise above810 C. thereby forming undesirable corrosion susceptible crystals in themicrospheres and the mechanical properties of articles containing thismaterial may be very poor.

The furnace was held at 775 C.:25 C. for sufiicient time to allowsubstantially all of the energy to be discharged in the form of heat(approximately three hours) and then cooled slowly to room temperature.The annealing time may vary depending on the size of the charge and thetemperature at which the energy discharge is carried out. For examplethe energy discharge which marks the energy level change of theparticles will occur more rapidly at 775 C. than at 300 C. Five gramsamples of the annealed powder were pressed in a /2 inch diametercylindrical mold at pressures of 50,000 psi. and 150,000 p.s.i. Compactgreen strength was measured in the same manner as in Example 1. Theresults are set forth in Table III.

TABLE IIL-EFFECT OF ANNEALING ON COMPACTI- I SILITY OF FUSED ANDATOMIZED ZIRCONIUM ALLOY POWDER Compacting pressure, p.s.i.: Percentcompacted 50,000 99+ 150,000 99+ Although the examples of thisspecification are directed toward fused and atomized powders ofzirconium and its alloys it is within the scope of this invention toinclude the high melting metals of Groups 4, 5, and 6, the rare earths,the actinium series and their alloys, which are only moderatelycompactible when formed into a powder by fusion and atomization.

While the invention has been described in connection with specificembodiments thereof, it will be understood that it is capable of furthermodification, and this application is intended to cover any varations,uses, or adaptations of the invention. It will, therefore, be recognizedthat the invention is not to be considered as limited to the preciseembodiments shown and described but is to be interpreted as broadly aspermitted by the appended claims.

We claim:

1. A method for producing highly compactible metal powder from fused andatomized particles of metals selected from a group consisting of Groups4, 5, 6, the rare earths, and the actinium series of the Periodic Tableand their alloys which comprises reducing said metal particles from ahigh energy state to a lower energy state as evidenced by the release ofenergy in the form of heat whereby the particles are readily compactibleinto compacts having high green strength, wherein said particles arereduced from said high energy state to said lower energy state by aplurality of compacting and recrushing steps.

2. A method for producing highly compactible metal powder from fused andatomized particles of metals selected from a group consisting of Groups4, 5, 6, the rare earths, and the actinium series of the Periodic Tableand their alloys which comprises reducing said metal particles from ahigh energy state to a lower energy state as evidenced by the release ofenergy in the form of heat whereby the particles are readily compactibleinto compacts having high green strength, wherein said particles arereduced from a high energy state to a lower energy state by acombination of steps comprising heating said powder under inertconditions to a temperature of at least 300 C. and compacting andcomminuting said powder at least once whereby said powder is madecompactible.

3. The method of claim 1 comprising the steps of compacting said powderat a pressure of at least 50,000 psi. and comminuting said compactedpowder, repeating said compacting and comminuting steps at least oncethereby reducing said particles of said powder from said high energystate to said lower energy state.

4. A highly compactible metal powder comprising fused and atomizedparticles of metal selected from a group consisting of Groups 4, 5, 6,the rare earths, and the actinium series and their alloys, said powderparticles having been produced by the method of claim 1.

References Cited UNITED STATES PATENTS OTHER REFERENCES ModernDevelopments in Powder Metallurgy, vol. 2, p. 269, John Googin et al.

HYLAND BIZOT, Primary Examiner W. W. STALLARD, Assistant Examiner U.S.Cl. X.R.

